Re: [PATCH v2] tmpfs: fix race between umount and writepage

[Hugh Dickins <hughd@google.com>]

On Tue, May 10, 2011 at 2:52 AM, Konstantin Khlebnikov
<khlebnikov@parallels.com> wrote:
> Hugh Dickins wrote:
>>
>> On Sun, 8 May 2011, Konstantin Khlebnikov wrote:
>>>
>>> Ok, I can test final patch-set on the next week.
>>> Also I can try to add some swapoff test-cases.
>>
>> That would be helpful if you have the time: thank you.
>
> I Confirm, patch 1/3 really fixes race between writepage and umount, as
> expected.

Good, thank you (but that path was identical to what you'd already tested).

>
> In patch 2/3: race-window between unlock_page and iput extremely small.

(I should clarify that the main race window is actually much wider
than that.  That page lock is only effective at holding off
shmem_evict_inode() while the page is in the file's pagecache -
between the (old positioning of) mutex_unlock(&shmem_swaplist_mutex)
and the add_to_page_cache_locked(), the page is just in swapcache and
so not recognizably attached to the file: shmem_evict_inode() will
call shmem_truncate_range(), and that would find the swp_entry_t, but
it frees it with a free_swap_and_cache() - which does not wait if it
cannot trylock the page.)

> My test works fine in parallel with thirty random swapon-swapoff,
> but it works without this patch too, thus I cannot catch this race.

Thanks for trying.  Given my difficulty in reproducing your umount
case, I'm not at all surprised that you didn't manage to reproduce
this swapoff case.  Indeed, I didn't even try to reproduce it myself:
I just saw the theoretical possibility once you'd warned me of
igrab(), and tested that this igrab-less approach works as well as the
old approach, without risking that race.

>
> I apply patch 3/3 too, but have not tested this case.

Fine, that part I could reproduce fairly easily for myself, and the
fix tested out fine.

Thanks,
Hugh

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